Colossians 3:5
Therefore put to death your members which are on the earth: fornication, uncleanness, passion, evil desire, and covetousness, which is idolatry.
Ephesians 5:3
But fornication and all uncleanness or covetousness, let it not even be named among you, as is fitting for saints;
Ephesians 5:2-4 (in Context) Ephesians 5 (Whole Chapter)
Galatians 5:19
Now the works of the flesh are evident, which are: adultery, fornication, uncleanness, lewdness,
Matthew 15:19
For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies.
fornication:
1. voluntary sexual intercourse between two unmarried persons or two persons not married to each other.
In these passages sleeping with someone outside of marriage is compared other sins, some "worse" then others. (however in the eyes of God according to the Word all sin is created equal)
Also, strictly world view, stds, unplanned pregnancy, feelings of deep depression after sleeping with someone who "loved" you deeply, to find out they were just trying to get into your pants...all very good reasons to abstain until marriage. That said, it is your life, and only God can judge you...